Yesterday I made like a mad dog or Englishman and measured a course in the midday sun. Calibration and the ride were done in 32C temps. On the way home, I decided to measure another course. The sun was sinking in the West, and the temp was down to 29-30C during the course of the ride. By the time I got home to post-cal, the temp had dropped to 27C.

Naturally, the post-cal constant was larger.

My question is, what should I use for the constant for each ride? The only "legal" solution is to use the average for the first course, but I think that will be less accurate. For the second course the average is OK, but might be a bit too generous, so I may use the larger constant.

FWIW, the second course was a remeasurement of an expired course, so I know what the actual distance should be.

My short answer: I just measure, but I would bite the bullet and remeasure on a better day at a better time.

The course measurement procedures for bikes with tubed, non-pressure regulated tires, while not explicit, are clearly biased toward a measurement process that takes place:

1) during static or warming temperatures (to maximally ensure the 2nd constant doesn't blow the 1st one out of the water).

2) with either mostly cloudy or completely sunny weather. Partly cloudy simply won't do, if done after mid-morning.

I relayed my recent experience to Jay Wight about having to postpone and redo a lowly calibration ride during partly cloudy conditions. I had temperature variances of 5C during the calibration that caused the longest and shortest counts for a 1/4 mile calibration ride to be off by 3 counts. This is near to or exceeding the 0.08% maximum allowed variance between 2 measurement rides!

Prepare and offer your ritual sacrifice to the weather gods, confirm that your sacrifice has been accepted, then go forth and measure in confidence.